This test verifies the support for gamma correction of displayed PNG
images. Several different images are displayed one above the other;
if gamma correction is correctly performed based on the values in
the gAMA chunk in the PNG file, the resulting displayed values are
the same in all of the files (except for rounding error, which gives
some artefacts at the right side of the lowest two images due to the
very high levels of gamma correction needed for this test).